In relatively recent times bicycle gearshifts with motorised actuation have been marketed, wherein the movement of the derailleur takes place through a driving member that is suitably controlled, typically electrically.
In the development of these gearshifts, a great deal of attention has obviously been given to the quality of the actuation, intended as the ability of the gearshift to carry out gear-shifting quickly and precisely at least to the same degree as a conventional manually-actuated gearshift.
However, even in normal use, the performance of a gearshift with motorised actuation is often compromised by even minor damage, which is often the result of knocks received by the gearshift itself.
Indeed, in gearshifts of this type the actuation kinematic mechanism is a system mechanically fixed to the driving member. In the case of knocks, due for example to the bicycle falling down or even to manoeuvres of loading, unloading and transportation on a vehicle, the gearshift can suffer damage of various degrees. A particularly severe knock can in the most serious cases cause some members of the gearshift to be broken; less severe knocks can on the other hand cause small deformations or even just small mutual displacements of the members of the gearshift, often not even immediately noticeable to the eye, but nevertheless such as to compromise the precision and therefore the correct operation of the gearshift. The same deformations or mutual displacements can in other cases be caused during gear-shifting by temporary locking of the kinematic mechanism with respect to the driving member, as sometimes occurs following mechanical interference between the chain and the sprockets (particularly when passing from one sprocket to another having a greater diameter).
Therefore, actuator devices have been developed which are provided with systems for protecting the gearshift against knocks or undesired temporary deformations/displacements. Such knocks or undesired temporary deformations/displacements are, for the sake of brevity, indicated hereafter with the expression “excess forces”.
One prior art solution an actuator device for a motorised gearshift wherein the motion provided by the motor is transferred to the actuation kinematic mechanism through a plurality of gear wheels. In order to provide protection against possible excess forces, a clutch device is provided between the actuation kinematic mechanism and the motor, the clutch device consisting of a driving member fixedly connected to the shaft of the motor, a driven member fixedly connected to the actuation kinematic mechanism and a torsion spring that pushes the driven member against the driving member. The driven member comprises a tooth that, in normal operating conditions, is housed in a seat formed in the driving member. In the presence of an excess force, a rotation force is generated on the driven member that exceeds a predetermined threshold. Such a force causes a relative rotation between driven member and driving member and the consequent slipping of the tooth out of the seat. The excess force is thus absorbed by the system thus preventing it from being transferred to the members of the actuation kinematic mechanism. The coupling surfaces of the tooth and of the driving member are shaped so that, when the tooth is out of the seat, there is a sufficiently large mutual contact area. This is to allow the normal operation of the gearshift even in this operative condition. The Applicant has observed that the plurality of gear wheels between motor and actuation kinematic mechanism makes the suggested device particularly complicated from the constructive point of view, as well as excessively bulky.
An improved actuator having elastically yielding protecting against excess forces is disclosed in Applicant's recently issues U.S. Pat. No. 8,033,937. In such an actuator device, the actuation kinematic mechanism has a nut associated therewith and the nut in turn is directly engaged with the drive shaft. The nut is provided with a preloaded spring arranged coaxially to the drive shaft. In a first operative configuration of the actuator device, the nut is in an engaged condition with the drive shaft such that the rotation of the drive shaft causes the deformation of the actuation kinematic mechanism. In the presence of an excess force, the spring deforms absorbing the excess force, thus avoiding it from being transferred to the members of the actuation kinematic mechanism. The spring is associated, at an end thereof, with a collar that, under a particularly high excess force, yields.
The Applicant has observed that in an actuator device of the type described above the breaking of the collar under a particularly high excess force causes the need to replace the nut, as well as the risk of damaging the members of the gearshift and/or of the bicycle due to the presence of mechanical pieces, like the collar and the spring, which are free to move without any constraint.
The Applicant has also observed that, in the actuator device described above, the spring, although deforming, still continues to exert a reaction to the excess forces. Therefore, under an excess force lasting a long time, the members of the actuation kinematic mechanism still remain subjected to a stress given by the resultant between the excess force and the reaction of the spring.
The technical problem addressed by the present invention is the provision of an actuator device that is constructively simple and with low bulk, provided with an effective system for protecting against knocks and that does not have the aforementioned drawbacks.